The present invention resides generally in the field of the preparation and use of 4-substituted pyridine compounds, and in particular to novel forms of supernucleophilic 4-substituted pyridine catalysts, and nucleophilic substitution processes useful for preparing such catalysts and other 4-substituted pyridines.
As further background, it is well known that many pyridines carrying an amino (desirably tertiary amino) group at the 4-position possess supernucleophilic properties making them highly advantageous for use as catalysts in acylation and other reactions. For example, the compound 4-N,N-dimethylaminopyridine (DMAP) is used on a large scale worldwide for acylation and other reactions in the pharmaceutical and agricultural industries. Historically, the preparation of 4-substituted pyridines such as DMAP has presented several challenges.
For example, tremendous research efforts worldwide have been made to discover effective means for transforming one group at the 4-position of the pyridine ring for another. Early on, researchers were hopeful that direct exposure of the free pyridine base to appropriate reagents would result in the effective modification of the 4-position. It has turned out, however, that most modifications of interest at the 4-position occur only at the cost of extreme conditions. For instance, 2-bromopyridine can be converted to 2-aminopyridine by reaction with ammonium hydroxide, but only at high temperatures of 200xc2x0 C. and under pressure. Den Hertog et al., Rec. Trav. Chim., 51, 381 (1932). Similarly, dimethylamine reacts with 4-chloropyridine only under pressure and at a temperature of 150xc2x0 C. (L. Pentimalli, Gass. Chem. Ital., 94, 902 (1964)), a process unsuitable for commercial scale. Likewise unsuitable for commercial scale is the reaction of sodium or potassium amide and metal methylanilides in etheral solvents or liquid ammonia, as described in Hauser, J. Org. Chem., 15, 310 (1949). N-pyridyl-4-pyridinium chloride hydrochloride or 4-phenoxypyridine has been reacted with nucleophiles to displace at the 4-position (D. Jerchel et al., Chem. Ber., 91, 1266 (1958)). However, these starting pyridine materials are far removed from commerce and thus such processes would be problematic if contemplated on a large scale.
In light of the difficulties of 4-substitution directly on the free base pyridine, a number of processes have been developed in which the 4-position (or 2-position) of the pyridine ring is activated toward nucleophilic substitution by a modification of the ring nitrogen of the pyridine. Such processes are generally known as activation-substitution-deactivation processes, and to date have involved either the N-oxidation or quaternization of the pyridine substrate, both of which are known to activate the 2- and 4-ring positions toward nucleophilic attack and expulsion of leaving groups at these positions. N-oxidation as a means to activate the 2- and 4-ring positions of pyridine has been relatively less studied than quaternization. This may be due to the fact that the level of activation imparted by N-oxidation is lower than that of quaternization. In the latter field, it is known that 4-substituted-pyridines such as 4-cyanopyridine can be quaternized with an alkyl iodide (e.g. methyl iodide) and reacted with ammonia to form a corresponding 4-aminopyridine. Metzger et al., J. Org. Chem., 41 (15), 2621 (1978). The dequaternization of such alkyl quats, however, is problematic, as only relatively exotic reagents such as triphenylphosphene/dimethylformamide (Aumann et al., J. Chem. Soc. Chem. Commun., 32, (1973)), triphenylphosphene/acetonitrile (Kutney et al., Synth. Commun., 5 (2), 119 (1975)) and diazabicyclononane/dimethylformamide or thiourea (Ho, Synth. Commun., 3, 99 (1973)) having been reported, with each of these processes inviting significant difficulty on an industrial scale.
More recently, research efforts have yielded quaternary-activated 4-substitution processes which can be practiced with greater advantage on a commercial scale. For example, U.S. Pat. No. 4,158,093 to Bailey et al. describes a route in which a 4-substituted pyridine base is quaternized with 2- or 4-vinylpyridine in the presence of a strong acid to yield a pyridylethyl quaternary salt. This activated quat form can then be subjected to nucleophilic substitution at the 4-position, and subsequently dequaternized in the presence of caustic.
U.S. Pat. Nos. 4,672,121 and 4,772,713 both to Nummy describe processes in which the 4-substituted pyridine base is reacted with acrylamide or an alkylacrylamide as the quaternizing reagent, and the resulting carbamoyl quat or a derivative therefrom is subject to nucleophilic displacement at the 4-position, again followed by dequaternization. In these ""121 and ""713 patents, the quaternization is conducted in the presence of a strong acid, and the substitution and dequaternization are conducted in the presence of a strong base such as alkali metal hydroxides or carbonates, or strong amidine bases.
The above-described research efforts have culminated in the past decade-and-a-half in the successful commercialization and worldwide use of the supernucleophilic catalyst, DMAP, and have opened the door to routes to similar useful 4-substituted pyridine compounds. However, needs remain for novel and improved 4-substitution processes for pyridines, and improved product forms. Desirable processes would entail the use of readily-available starting materials and reagents while providing high purity products and minimizing and/or simplifying purification steps. Improved processes would also minimize reagent use and the need to recycle materials or handle or dispose hazardous wastes. As well, new product forms, especially of supernucleophilic 4-substituted pyridine catalysts, would avoid or reduce difficulties which have been encountered in the handling of crystalline or flaked catalyst forms which have been available to date. The present invention provides several embodiments, each of which addresses one or more of these needs.
Accordingly, one feature of the present invention is the provision of a supernucleophilic 4-substituted pyridine catalyst in a unique form, and a process for making the same. The preferred process for preparing a granular supernucleophilic 4-substituted pyridine catalyst, especially a monoalkylamino- or dialkylaminopyridine catalyst, includes a step of providing the supernucleophilic catalyst as a molten flowable mass. This flowable mass is then extruded through an orifice into discrete liquid portions each corresponding to a granule to be formed. These liquid portions, in turn, are cooled to form a granular supernucleophilic catalyst. The granular supernucleophilic catalyst, most preferably 4-N,N-dimethylaminopyridine (DMAP), desirably has an average particular diameter of about 1 to about 10 mm. Suitable melt temperatures range from the melting point for the catalyst, e.g. 111-112xc2x0 C. or DMAP, up to just below the decomposition temperature for the catalyst, with preferred melt temperatures ranging from about the melting point of the catalyst up to about 50xc2x0 above that point, e.g. for DMAP about 112xc2x0 C. to about 160xc2x0 C., more preferably from the melting point up to about 30xc2x0 C. above the melting point, and especially for DMAP about 115xc2x0 C. to about 130xc2x0 C.
In still more preferred processes, the extruding step is conducted using equipment optimally designed for forming the discreet portions. For example, such may involve an extrusion apparatus equipped to deliver the flowable mass through an orifice for a predetermined period of time to provide drops of the appropriate size. This control can be achieved, for example, by providing first and second wall members each having orifices, wherein the wall members are movable relative to one another to periodically align orifices in the first member with those in the second member for the predetermined period of time. The flowable mass is pressurized against the first wall member such that when the orifices in the first and second wall member are aligned, an amount of the flowable mass is extruded through the aligned orifices, for example downwardly onto a conveyor belt. Most preferred devices for these purposes include as the first member, a first container, e.g. a drum, filled and pressurized with the flowable mass, and as the second member a second container, e.g. a second drum, encasing the first container. Each container has orifices, and they are movable (e.g. rotatable) with respect to one another (preferably provided by a static inner container and a movable (rotating) outer container. Movement of the second container results in periodic alignment of the orifices for the predetermined time, during which the drops of supernucleophilic catalyst material are extruded through the aligned orifices and downwardly onto a passing conveyer. Such processes provide preferred, smooth-surfaced supernucleophilic catalyst granules of uniform size and shape, for example generally hemispherical in shape.
Another preferred embodiment of the invention provides a catalyst composition comprising a granulated supernucleophilic 4-(secondary or tertiary)aminopyridine catalyst, especially a dialkylaminopyridine catalyst such as DMAP. Preferred such catalysts have an average particle diameter of about 1 mm to about 10 mm, with most preferred catalyst forms having smooth granules of substantially uniform size and/or shape.
Additional preferred embodiments of the invention relate to improved activation-substitution-deactivation routes to 4-substituted pyridines. On such preferred embodiment involves a process for preparing a 4-(secondary or tertiary)aminopyridine compound. This process includes reacting a starting 4-substituted pyridine base having a leaving group as the 4-substituent, with an activating agent of the formula: 
wherein R3 and R4, which may be the same as or may differ from one another, are each xe2x80x94H or a C1-C4 alkyl group, and Z is xe2x80x94OR7 or NR5R6, wherein R5 and R6, which may be the same as or may differ from one another, and may taken together form a ring, are each xe2x80x94H or C1-C8 alkyl; and R7 is xe2x80x94H or C1-C8 alkyl. This reacting forms an activated 1,4-substituted pyridine, which is then reacted with a primary or secondary amine in at least a 3:1 molar ratio relative to the pyridine, to form a corresponding 1-substituted, 4-(secondary or tertiary)aminopyridine. The 1-substituted, 4-(secondary or tertiary)aminopyridine is then treated to remove the 1-substituent and thereby form a product medium including the 4-(secondary or tertiary)aminopyridine. It has been found that by conducting the substitution step in the presence of a large molar excess of the amine used as the nucleophile in the substitution, the use of strong bases such as alkali metal hydroxides in the substitution step can be minimized or eliminated, and that downstream product separations are simplified, providing highly pure, white 4-(secondary or tertiary) aminopyridine products even absent a solvent recrystallization step. This process is applied with preference to a manufacture of DMAP, wherein the amine is dimethylamine. The activating agent in this process is preferably acrylic acid or acrylamide.
Another embodiment of the present invention provides an activation-substitution-deactivation route to 4-nucleophile-substituted pyridines, wherein the activated pyridine species is a pyridine betaine. Preferred processes include reacting a starting 4-substituted pyridine base having a leaving group as the 4-substituent, with an xcex1,xcex2-unsaturated acid of the formula 
wherein R3 and R4, which may be the same as or may differ from one another, are each xe2x80x94H or a C1-C4 alkyl group, so as to form a corresponding activated 1,4-substituted pyridine betaine. The betaine is reacted with a nucleophile (Nu) to displace the leaving group and form a 1-substituted, 4-Nu-pyridine betaine. This betaine is then treated to remove the 1-substituent from the 4-Nu-pyridine compound. Preferred processes of this embodiment involve activation steps conducted in the absence of acid other than the xcex1,xcex2-unsaturated acid, and further the nucleophilic substitution is optionally conducted under mild basic conditions (i.e. in the absence of strong bases such as alkali metal hydroxide) in the presence of a primary or secondary amine used as the nucleophile in at least a 3:1 molar ratio relative to the pyridine betaine. In its most desirable form to date, this process involves the reaction of 4-cyanopyridine with acrylic acid to form a corresponding betaine. This betaine is reacted with dimethylamine to form a corresponding 4-N,N-dimethylaminopyridine betaine. This betaine is then treated in the presence of a strong base such as sodium hydroxide to remove the 1-substituent and form DMAP.
A still further embodiment of the invention provides a novel, optionally isolated, pyridine betaine of the formula 
wherein:
G is a group selected from xe2x80x94CN and xe2x80x94NR1R2, wherein R1 and R2, which may be the same or may differ from one another, are each xe2x80x94H or a hydrocarbon group having from one to about ten carbon atoms, especially C1-C10 alkyl groups, and most preferably methyl groups; and
R3 and R4, which may be the same as or may differ from one another, are selected from xe2x80x94H and C1-C4 alkyl groups.
A still further preferred embodiment of the invention provides heat stable 4-(secondary or tertiary)aminopyridine catalysts which may be produced by processes of the invention. Such heat stability can be exhibited in one or more of several ways. For example, preferred products, especially DMAP products, have an APHA color of less than about 50 and exhibit an increase in APHA color of no greater than about 50 when heated in a nitrogen atmosphere at about 120xc2x0 C. for about 24 hours. For instance, more preferred DMAP products have an APHA color of less than about 10, and exhibit an APHA color of no greater than about 50 after heating in a nitrogen atmosphere at about 120xc2x0 C. for about 24 hours. In another feature demonstrating heat stability, the present invention provides amorphous (i.e. non-crystalline form) 4-(secondary or tertiary)aminopyridine catalysts, particularly DMAP catalysts, having an APHA color of less than 20, more preferably less than 10.
The invention provides improved supernucleophilic catalysts and improved synthetic routes which can be used to prepare such catalysts and other useful substituted pyridines. The novel catalyst forms overcome handling and processing difficulties previously encountered with supernucleophilic catalysts, and preferred processes can be used to provide high yields while employing readily available materials, minimizing the use of reagents, and/or minimizing the difficulty and/or number of product purification steps. Additional objects, features and advantages of the invention will be apparent from the description that follows.